Solar cells that provide an environment-friendly electric energy source have drawn public attention as an effective energy source that can solve energy problems that have currently become more and more serious. At present, as a semiconductor material for use in photovoltaic devices for solar cells, inorganic substances such as monocrystalline silicon, polycrystalline silicon, amorphous silicon, and a compound semiconductor, have been used. However, since the solar cell to be produced by using inorganic semiconductors requires high costs, it has not been widely used for general household purposes. The main reason for the high costs lies in that a process of manufacturing a semiconductor thin-film requires high temperature and vacuum conditions. For this reason, organic solar cells have been investigated in which, as a semiconductor material expected to simplify the manufacturing process, an organic semiconductor and an organic dye such as a conjugated polymer and an organic crystal are used.
However, the largest problem with the organic solar cells using the conjugated polymer or the like is that its photoelectric conversion efficiency is low compared to conventional solar cells using inorganic semiconductors, and these solar cells have not been put into practical use. The reasons that the photoelectric conversion efficiency of the organic solar cells using the conventional conjugated polymer is low lie in that the absorbing efficiency of solar light is low, in that a bound state referred to as an exciton state in which electrons and holes generated by solar light are hardly separated is formed, and in that since a trap which captures carriers (electrons and holes) is easily formed, generated carriers are easily captured by the trap, resulting in the slow mobility of carriers.
At present, the conventional photoelectric conversion device based on the organic semiconductors can be classified into a schottky-type structure in which an electron donating organic material (p-type organic semiconductor) and metal having a small work function are joined to each other, and a hetero junction type structure in which an electron accepting organic material (n-type organic semiconductor) and an electron donating organic material (p-type organic semiconductor) are joined to each other. These devices have a low photoelectric conversion efficiency since only the organic layer (only several molecular layer) of the joined portion contributes to photoelectric current generation, and the improvement thereof has been required.
As a method of improving the photoelectric conversion efficiency, there is a method of employing a bulk hetero junction type structure in which an electron accepting organic material (n-type organic semiconductor) and an electron donating organic material (p-type organic semiconductor) are mixed with each other to increase the junction surface contributing to the photoelectric conversion. In particular, a photoelectric conversion device of a bulk hetero junction type has been reported in which a conjugated polymer is used as the electron donating organic material (p-type organic semiconductor), and a conductive polymer having n-type semiconductor characteristics, fullerene such as C60 or a fullerene derivative, is used as the electron accepting organic material.
By the way, to efficiently absorb radiating energy which covers a wide range of solar light spectra to improve the photoelectric conversion efficiency, an electron donating organic material with a narrow band gap is useful (for example, refer to E. Bundgaard and F. C. Krebs, “Solar Energy Materials & Solar Cells”, Vol. 91, p. 954, 2007 and H. Zhou, L. Yang, and W. You, “Macromolecules”, Vol. 45, p. 607, 2012). It is reported that as such a narrow-band-gap electron donating organic material, a copolymer formed by combining a thieno[3,4-b]thiophene skeleton with a benzo[1,2-b:4,5-b′]dithiophene skeleton exhibits particularly excellent photovoltaic characteristics, and many derivatives have been synthesized (for example, refer to WO 2011/011545 A).
However, in conventional electron donating organic materials formed by copolymerization of the thieno[3,4-b]thiophene skeleton and the benzo[1,2-b:4,5-b′]dithiophene skeleton, sufficient conversion efficiency is not achieved since it is not possible to pursue narrowing of a band gap, high carrier mobility and compatibility with the electron accepting material typified by the fullerene derivatives simultaneously. It could therefore be helpful to provide an electron donating organic material which pursues narrowing of a band gap, high carrier mobility and compatibility with the electron accepting material simultaneously by selecting an optimum substituent and side chain, and provide a photovoltaic device having high photoelectric conversion efficiency.